gods-eye-view/server/providers/overpass/geometry.js

import {
  OVERPASS_SIMPLIFY_MIN_BYTES,
  OVERPASS_SIMPLIFY_MIN_POINTS,
  OVERPASS_SIMPLIFY_TOLERANCE_DEG,
} from './constants.js';

/** Iterative Douglas-Peucker on [{lat,lon},...] (planar-degree approx — fine at
 *  the ~44 m tolerance used here). Endpoints always kept. */
function douglasPeucker(points, toleranceDeg) {
  const n = points.length;
  if (n <= 2) return points;
  const keep = new Uint8Array(n);
  keep[0] = 1;
  keep[n - 1] = 1;
  const stack = [[0, n - 1]];
  while (stack.length) {
    const [a, b] = stack.pop();
    if (b - a < 2) continue;
    const ax = points[a].lon;
    const ay = points[a].lat;
    const vx = points[b].lon - ax;
    const vy = points[b].lat - ay;
    const c2 = vx * vx + vy * vy;
    let worst = -1;
    let worstDist = toleranceDeg;
    for (let i = a + 1; i < b; i++) {
      const wx = points[i].lon - ax;
      const wy = points[i].lat - ay;
      let d;
      if (c2 === 0) {
        d = Math.hypot(wx, wy);
      } else {
        const t = Math.max(0, Math.min(1, (vx * wx + vy * wy) / c2));
        d = Math.hypot(wx - t * vx, wy - t * vy);
      }
      if (d > worstDist) {
        worstDist = d;
        worst = i;
      }
    }
    if (worst >= 0) {
      keep[worst] = 1;
      stack.push([a, worst], [worst, b]);
    }
  }
  const out = [];
  for (let i = 0; i < n; i++) if (keep[i]) out.push(points[i]);
  return out;
}

/** Simplify one element's geometry array in place if it is big enough. */
function simplifyElementGeometry(el, minPoints, toleranceDeg) {
  if (Array.isArray(el?.geometry) && el.geometry.length >= minPoints) {
    el.geometry = douglasPeucker(el.geometry, toleranceDeg);
  }
  if (Array.isArray(el?.members)) {
    for (const member of el.members) {
      if (
        Array.isArray(member?.geometry) &&
        member.geometry.length >= minPoints
      ) {
        member.geometry = douglasPeucker(member.geometry, toleranceDeg);
      }
    }
  }
}

/**
 * Server-side geometry simplification for large Overpass `out geom` payloads.
 * Region/state boundary pivots return multi-MB coastline rings whose fidelity
 * nothing downstream needs (the client re-simplifies for draw); decimating them
 * HERE shrinks the disk cache, the wire, and client parse time — and is what
 * makes the raised read cap safe. Anything unparseable or below the thresholds
 * passes through byte-identical. Exported for tests (opts override thresholds).
 *
 * @param {string} bodyText - Raw upstream JSON body.
 * @returns {string} Possibly-simplified JSON body.
 */
function simplifyOverpassPayloadBody(bodyText, opts = {}) {
  const minBytes = opts.minBytes ?? OVERPASS_SIMPLIFY_MIN_BYTES;
  const minPoints = opts.minPoints ?? OVERPASS_SIMPLIFY_MIN_POINTS;
  const toleranceDeg = opts.toleranceDeg ?? OVERPASS_SIMPLIFY_TOLERANCE_DEG;
  if (typeof bodyText !== 'string' || bodyText.length < minBytes)
    return bodyText;
  let data;
  try {
    data = JSON.parse(bodyText);
  } catch {
    return bodyText;
  }
  if (!Array.isArray(data?.elements)) return bodyText;
  for (const el of data.elements)
    simplifyElementGeometry(el, minPoints, toleranceDeg);
  try {
    return JSON.stringify(data);
  } catch {
    return bodyText;
  }
}

export { simplifyOverpassPayloadBody };